The invention relates to an iron-chromium-aluminum alloy having a long service life and exhibiting little change in heat resistance, which is produced by way of fusion metallurgy.
Iron-chromium-aluminum-tungsten alloys are used to produce electric heating elements and catalyst carriers. These materials form a dense, firmly adhering aluminum oxide layer, which protects them from damage at high temperatures (for example up to 1400° C.). This protection is improved by the addition of in the range of 0.01 to 0.3% of so-called reactive elements, such as Ca, Ce, La, Y, Zr, Hf, Ti, Nb and W, which, among other things, improve the adhesive strength of the oxide layer and/or the layer growth, as is described, for example in “Ralf Bürgel, Handbuch der Hochtemperatur-Werkstofftechnik (Handbook of High-Temperature Materials Technology), Vieweg Publishing House, Braunschweig 1998”, starting on page 274.
The aluminum oxide layer protects the metallic material from rapid oxidation. In the process, the layer itself grows, albeit very slowly. This growth takes place while consuming the aluminum content of the material. When aluminum is no longer present, other oxides (chromium and iron oxides) grow, and the metal content of the material is consumed very quickly, so that the material fails due to destructive corrosion. The time until failure is referred to as the service life. Increasing the aluminum content extends the service life.
In all of concentration information in the specification, as well as in the patent claims, % denotes information in percentage by weight.
From WO 02/20197 A1 a ferritic stainless steel alloy is known, particularly for use as a heating element. The alloy is formed by a powder metallurgically produced Fe—Cr—Al alloy, comprising less than 0.02% C, ≦0.5% Si, ≦0.2% Mn, 10.0 to 40.0% Cr, ≦0.6% Ni, ≦0.01% Cu, 2.0 to 10.0% Al, one or more element(s) of the group of reactive elements such as Sc, Y, La, Ce, Ti, Zr, Hf, V, Nb, Ta, at levels ranging between 0.1 and 1.0%, and a remainder of iron and unavoidable impurities.
DE 199 28 842 A1 describes alloy comprising 16 to 22% Cr, 6 to 10% Al, 0.02 to 1.0% Si, a maximum of 0.5% Mn, 0.02 to 0.1% Hf, 0.02 to 0.1% Y, 0.001 to 0.01% Mg, a maximum of 0.02% Ti, a maximum of 0.03% Zr, a maximum of 0.02% SE, a maximum of 0.1% Sr, a maximum of 0.1% Ca, a maximum of 0.5% Cu, a maximum of 0.1% V, a maximum of 0.1% Ta, a maximum of 0.1% Nb, a maximum of 0.03% C, a maximum of 0.01% N, a maximum of 0.01% B, and a remainder of iron and steel production-related impurities, for the use as a carrier foil for exhaust gas catalysts, as a heating element, and as a component in industrial furnace construction and in gas burners.
EP 0 387 670 B1 describes an alloy comprising (in % by weight) 20 to 25% Cr, 5 to 8% Al, 0.03 to 0.08% yttrium, 0.004 to 0.008% nitrogen, 0.020 to 0.040% carbon, and approximately equal amounts of 0.035 to 0.07% Ti and 0.035 to 0.07% zirconium, and a maximum of 0.01% phosphorus, a maximum of 0.01% magnesium, a maximum of 0.5% manganese, a maximum of 0.005% sulfur, the remainder being iron, wherein the sum of the contents of Ti and Zr is 1.75 to 3.5% times as great as the sum, as a percentage, of the contents of C and N, and steel production-related impurities. Ti and Zr can be partially or completely replaced with hafnium and/or tantalum or vanadium.
EP 0 290 719 B1 describes an alloy comprising (in % by weight) 12 to 30% Cr, 3.5 to 8% Al, 0.008 to 0.10% carbon, a maximum of 0.8% silicon, 0.10 to 0.4% manganese, a maximum of 0.035% phosphorus, a maximum of 0.020% sulfur, 0.1 to 1.0% molybdenum, a maximum of 1% nickel and the additions of 0.010 to 1.0% zirconium, 0.003 to 0.3% titanium and 0.003 to 0.3% nitrogen, 0.005 to 0.05% calcium plus magnesium, as well as 0.003 to 0.80% rare earth metals, 0.5% niobium, the remainder being iron including incidental impurities, which is used, for example, as a wire for heating elements for electrically heated ovens, as a construction material for parts subject to thermal stress, and as a foil for producing catalyst carriers.
U.S. Pat. No. 4,277,374 describes an alloy comprising (in % by weight) up to 26% chromium, 1 to 8% aluminum, 0.02 to 2% hafnium, up to 0.3% yttrium, up to 0.1% carbon, up to 2% silicon, the remainder being iron, and preferred ranges being 12 to 22% for chromium and 3 to 6% for aluminum, which is used as a foil for producing catalyst carriers.
From U.S. Pat. No. 4,414,023 a steel is known, comprising (in % by weight) 8.0 to 25.0% Cr, 3.0 to 8.0% Al, 0.002 to 0.06% rare earth metals, and a maximum of 4.0% Si, 0.06 to 1.0% Mn, 0.035 to 0.07% Ti, 0.035 to 0.07% Zr, and including unavoidable impurities.
DE 10 2005 016 722 A1 discloses an iron-chromium-aluminum alloy having a long service life, comprising (in % by weight) 4 to 8% Al and 16 to 24% Cr, and additions of 0.05 to 1% Si, 0.001 to 0.5% Mn, 0.02 to 0.2% Y, 0.1 to 0.3% Zr and/or 0.02 to 0.2% Hf, 0.003 to 0.05% C, 0.0002 to 0.05% Mg, 0.0002 to 0.05% Ca, a maximum of 0.04% N, a maximum of 0.04% P, a maximum of 0.01% S, a maximum of 0.5% Cu, and the customary steel production-related impurities, the remainder being iron.
A detailed model of the service life of iron-chromium-aluminum alloys is described in the article by I. Gurrappa, S. Weinbruch, D. Naumenko, W. J. Quadakkers, Materials and Corrosion 51 (2000), on pages 224 to 235. The article highlights a model in which the service life of iron-chromium-aluminum alloys is said to be dependent on the aluminum content and the sample shape, wherein potential spalls are not taken into consideration in the formula (aluminum depletion model).
            t      B        =                  [                  4          ,                      4            ×                          10                              -                3                                      ×                          (                                                C                  0                                -                                  C                  B                                            )                        ×                                          ρ•                ⁢                                                                  ⁢                f                            k                                      ]                    1        n                        where      ⁢                          ⁢      f        =          2      ×              volume        surface            
tB=Service life, defined as the time until other oxides occur as aluminum oxide
CO=Aluminum concentration at the beginning of oxidation
CB=Aluminum concentration when other oxides occur as aluminum oxides
ρ=Specific density of the metallic alloy
k=Oxidation rate constant
n=Oxidation rate exponent
Taking the spalls into consideration, the following formula is obtained for a flat sample having infinite width and length and a thickness d (f≈d):
            t      B        =    4    ,      4    ×          10              -        3              ×          (                        C          0                -                  C          B                    )        ×    ρ    ×    d    ×          k              -                  1          n                      ×                  (                  Δ          ⁢                                          ⁢                      m            •                          )                              1          n                -        1            where Δm* is the critical weight change at which the spalling begins.
Both formulas show that the service life is shortened as the aluminum content decreases and when the surface-to-volume ratio is high (or the sample thickness is smaller).
This becomes significant when thin foils in the dimensional range of approximately 20 μm to approximately 300 μm must be used for specific applications.
Heat conductors that are made of thin foils (for example a thickness of approximately 20 to 300 μm with a width in the range of one to several millimeters) are characterized by a large surface-to-volume ratio. This is advantageous when fast heating and cooling times are to be achieved, for example those required for heating elements used in glass ceramic fields, so as to make heating visibly faster and to achieve quick heating similar to that with a gas stove. At the same time, however, the large surface-to-volume ratio is disadvantageous for the service life of the heating element.
When using an alloy as a heat conductor, the behavior of the heat resistance must also be taken into consideration. In general, a constant voltage is applied to the heat conductor. If the resistance remains constant over the course of the service life of the heating element, the current and power of this heating element are also unchanged.
However, given the processes described above, in which aluminum is continuously consumed, this is not the case. As a result of the consumption of aluminum, the specific electric resistance of the material decreases. However, this is done by removing atoms from the metallic matrix, which is to say the cross-section is reduced, which results in increased resistance (see Harald Pfeifer, Hans Thomas, Zunderfeste Legierungen [Scale-Proof Alloys], Springer publishing house, Berlin/Göttingen/Heidelberg/ 1963 page 111). Due to the stresses that develop as the oxide layer grows and the stresses resulting from the different coefficients of expansion of the metal and oxide when heating and cooling the heat conductor, additional stresses are created, which can result in a deformation of the foil and a consequent dimensional change (see also H. Echsler, H. Hattendorf, L. Singheiser, W. J. Quadakkers, Oxidation behaviour of Fe—Cr—Al alloys during resistance and furnace heating, Materials and Corrosion 57 (2006) 115-121). Depending on the interaction of the dimensional changes with the change in the specific electric resistance, an increase or a decrease in the heat resistance of the heat conductor may occur over the course of the usage. These dimensional changes become more significant with the number of times that the heat conductor is heated and cooled, that is, the length of the cycle. In the process, the foil is deformed in the manner of watch glass. This creates additional damage to the foil, so that this is another important failure mechanism in the very short and fast cycles of foils, which may even be decisive, depending on the cycle and temperature.
An increase in the heat resistance over time is generally observed for wires made of iron-chromium-aluminum alloys (Harald Pfeifer, Hans Thomas, Zunderfeste Legierungen [Scale-Proof Alloys], Springer Publishing House, Berlin/Göttingen/Heidelberg/1963 page 112) (FIG. 1), while a drop in heat resistance is generally observed for heat conductors in the form of foils made of iron-chromium-aluminum alloys (FIG. 2).
If the heat resistance RW rises over time, the power P decreases, with the voltage being kept constant, at the heating element that is produced therefrom, which is calculated with P=U*I=U2/RW. As the power at the heating element decreases, so does the temperature of the heating element. The service life of the heat conductor and therefore of the heating element is thereby extended. However, heating elements often have a lower limit for the power, so that this effect cannot be employed arbitrarily to extend the service life. If, in contrast, the heat resistance RW decreases over time, the power P increases at the heating element, with the voltage being kept constant. However, as the power increases, so does the temperature and, as a result, the service life of the heat conductor or heating element is shortened. This is intended to keep the variances of the heat resistance as a function of time within a narrowly limited range around zero.
The service life and the behavior of the heat resistance can be measured, for example, using an accelerated service life test. Such a test is described, for example, in Harald Pfeifer, Hans Thomas, Zunderfeste Legierungen [Scale-Proof Alloys], Springer Publishing House, Berlin/Göttingen/Heidelberg/1963, on page 113. The test is conducted using a switching cycle of 120 s, at a constant temperature, on wire that is shaped into helices having a diameter of 0.4 mm. Temperatures of 1200° C. and 1050° C. are proposed as the test temperatures. However, since specifically the behavior of thin foils is to be analyzed in this case, the test was modified as follows:
Foil strips measuring 50 μm in thickness and 6 mm in width were clamped between 2 current feed-throughs and heated to 1050° C. by applying a voltage. In each case, heating to 1050° C. was performed for 15 s, then the power supply was interrupted for 5 s. At the end of the service life, the foil failed in that the remaining cross-section thoroughly melted. The temperature is measured automatically during the service life test using a pyrometer and, where necessary, is corrected to the target temperature by a program controller.
The burning period is used as a measure of the service life. The burning period or burning time is the sum of the times during which the sample is heated. The burning period is the time until failure of the samples, while the burning time is the running time during an experiment. In all subsequent figures and tables, the burning period or the burning time is given as a relative value in %, relative to the burning period of a reference sample, and is referred to as the relative burning period or relative burning time.
From the prior art described above, it is known that minor additions of Y, Zr Ti, Hf, Ce, La, Nb, V, and the like heavily influence the service life of FeCrAl alloys.
The market places increased demands on products which require a longer service life and an increased usage temperature of the alloys.